Hey Jim,
On average, it takes 4 times longer, and 4 time more than you thought to do the machine, unless you're really experienced and doing this kind of stuff.
Yes, you can operate your machine via, Jog stick and MPG, it is a matter of bringing those signals in and doing something with them via: Brains, or MacroPump,
or a plugin.
First thing to do is see what is re-usable:
Check the Axis motors see what drives them, then find out if the drive that drives them is stand-a-lone drives, that will take a Step and Dir signal or Analog.
if step and dir, Mach3 and/or some external motion boards can drive them. If analog, then you gonna drop some cash on a Galil or the like.
Spindle Motor: if it is just an industrial motor (even if 3 phaze), you can use it, if your machine will be running off of 220v single phase, then you will need a VFD.
Where you get into MAJOR cash, is if you have to replace Axis motors and/or drives, or spindle motor/drives etc. Depending on the motor power you need (In/Ilbs, N/M etc.)
If you Have some pockets, AND you plan to make money with the machine, you will recoup your expense and be MUCH, MUCH more pleased if you just upgrade
your Axis motors to: AC Servo Drives/Motors (I use Automation Direct for 1kw-3kw, and Yaskawa beyond that). If your spindle motor is an industrial, it is defiantly worth your time to put a VFD to drive it, since it give you VERY precise output to the motor, (and you can set up things for like ATC's much easier with them also).
There are several companies that cater to the Mach3 crowd for retrofits/builds, these are just a few: CNC4PC, Homann Designs, PMDX.....etc.)
I use Automation Direct for almost ALL of my industrial supplies, since it is the "Walmart" version of industrial supplies for pricing.
Scott